1. Field of the Invention
Generally, the present disclosure relates to the field of fabricating products, such as semiconductor devices, in a manufacturing environment including process tools exchanging transport carriers with an automated transport system, wherein the products, such as substrates for semiconductor devices, are processed on the basis of groups or lots defined by the contents of the transport carriers.
2. Description of the Related Art
Today's global market forces manufacturers of mass products to offer high quality devices at a low price. It is thus important to improve yield and process efficiency to minimize production costs. This holds especially true in the field of semiconductor fabrication, since, here, it is essential to combine cutting-edge technology with volume production techniques. It is, therefore, the goal of semiconductor manufacturers to reduce the consumption of raw materials and consumables while at the same time improve process tool utilization. The latter aspect is especially important since, in modern semiconductor facilities, equipment is required which is extremely cost-intensive and represents the dominant part of the total production costs.
Integrated circuits are typically manufactured in automated or semi-automated facilities, by passing substrates comprising the devices through a large number of process and metrology steps to complete the devices. The number and the type of process steps and metrology steps a semiconductor device has to go through depends on the specifics of the semiconductor device to be fabricated. A usual process flow for an integrated circuit may include a plurality of photolithography steps to image a circuit pattern for a specific device layer into a resist layer, which is subsequently patterned to form a resist mask for further processes in structuring the device layer under consideration by, for example, etch or implant processes and the like. Thus, layer after layer, a plurality of process steps are performed based on a specific lithographic mask set for the various layers of the specified device. For instance, a sophisticated CPU requires several hundred process steps, each of which has to be carried out within specified process margins so as to fulfill the specifications for the device under consideration. Since many of these processes are very critical, such as many photolithography steps, a plurality of metrology steps have to be performed to efficiently control the process flow and to monitor the performance of the respective process tools. For example, so-called pilot substrates are frequently processed and subjected to measurement procedures prior to actually releasing the associated group of “parent” substrates in order to test the compliance with predefined process margins. Typical metrology processes may include the measurement of layer thickness, the determination of dimensions of critical features, such as the gate length of transistors, the measurement of dopant profiles, and the like. As the majority of the process margins are device-specific, many of the metrology processes and the actual manufacturing processes are specifically designed for the device under consideration and require specific parameter settings at the adequate metrology and process tools.
In a semiconductor facility, a plurality of different product types are usually manufactured at the same time, such as memory chips of different design and storage capacity, CPUs of different design and operating speed and the like, wherein the number of different product types may even reach one hundred and more in production lines for manufacturing ASICs (application specific ICs). Since each of the different product types may require a specific process flow, different mask sets for the lithography, specific settings in the various process tools, such as deposition tools, etch tools, implantation tools, chemical mechanical polishing (CMP) tools and the like may be necessary. Consequently, a plurality of different tool parameter settings and product types may be encountered simultaneously in a manufacturing environment. Thus, a mixture of product types, such as test and development products, pilot products, different versions of products, at different manufacturing stages, may be present in the manufacturing environment at a time, wherein the composition of the mixture may vary over time depending on economic constraints and the like, since the dispatching of non-processed substrates into the manufacturing environment may depend on various factors, such as the ordering of specific products, a variable degree of research and development efforts and the like. Thus, frequently, the various product types may have to be processed with a different priority to meet requirements imposed by specific economic or other constraints.
Despite these complex conditions, it is an important aspect with respect to productivity to coordinate the process flow within the manufacturing environment in such a way that high performance, for example in terms of tool utilization, of the process tools is achieved, since the investment costs and the moderately low “life span” of process tools, particularly in a semiconductor facility, significantly determine the price of the final semiconductor devices. In modern semiconductor facilities, a high degree of automation is typically encountered, wherein the transport of substrates from and to the process and metrology tools is accomplished on the basis of respective transport carriers accommodating a specific maximum number of substrates. The number of substrates contained in a carrier is also referred to as a lot and the number of substrates is therefore frequently called the lot size. In a highly automated process line of a semiconductor facility, the transport of the carriers is mainly performed by an automated transport system that picks up a carrier at a specific location, for example a load port associated with a process or metrology tool, within the environment and delivers the carrier to its destination, for instance a load port of another process or metrology tool that may perform the next process or processes required in the respective process flow of the products under consideration. Thus, the products in one carrier typically represent substrates to be processed in the same process tool, wherein the number of substrates in the carrier may not necessarily correspond to the maximum number of possible substrates. That is, the lot size of the various carriers may vary, wherein typically a “standard” lot size may dominate in the manufacturing environment. For example, one or more pilot substrates, which may be considered as representatives of a certain number of parent substrates contained in a certain number of carriers filled with the standard lot size, may be transported in a separate carrier, since they may undergo a specific measurement process and therefore may have to be conveyed to a corresponding metrology tool, thereby requiring an additional transport job. Based on the results of the measurement process, the waiting parent substrates may then be delivered to the respective process tool.
The supply of carriers to and from process tools is usually accomplished on the basis of respective “interfaces,” also referred to as loading stations or load ports, which may receive the carriers from the transport system and hold the carriers to be picked up by the transport system. The transport system comprises a rail system that is typically attached to the clean room ceiling so that the transport system is usually referred to as an overhead transport system (OHT). Furthermore, the OHT accommodates a plurality of vehicles running along the OHT rails in order to convey a transport carrier that is to be exchanged with a specific process tool by means of one or more load ports associated with the tool under consideration. Due to the increasing complexity of process tools, having implemented therein a plurality of functions, the cycle time for a single substrate may increase. Hence, when substrates are not available at the tool although being in a productive state, significant idle times or unproductive times may be created, thereby significantly reducing the utilization of the tool. Thus, typically, the number and configuration of the load ports is selected such that one or more carriers may be exchanged at the load port(s) while the functional module of the process tool receives substrates from another load port to achieve a cascaded or continuous operation of the functional module of the process tool. The time for the exchange of carriers between the automated transport system and the respective process or metrology tool depends on the transport capacity of the transport system and the availability of the carrier to be conveyed at its source location. Ideally, when a corresponding transport request for a specified lot currently processed in a source tool is to be served, the respective substrates should be available at the time the transport system picks up the carrier including the lot and delivers the carrier at the destination tool such that a continuous operation may be maintained. Consequently, the respective carrier should be delivered to the destination tool when or before the last substrate of the carrier currently processed in the destination tool is entered into the process module so that a continuous operation may be achieved on the basis of the newly arrived carrier. Thus, for an ideal continuous operation of a process tool, one carrier would be exchanged while another carrier is currently processed.
Depending on the capacity of the tool interface, for instance the number of load ports provided, a certain buffer of carriers and thus substrates may be provided in order to generate a certain tolerance for delays and irregular deliveries, which may, however, significantly contribute to tool costs. In some circumstances, the required carrier exchange time for maintaining a continuous operation of the tool under consideration may even be negative, thereby requiring a change of the substrate handling scenario. Moreover, the actual carrier exchange time, i.e., the time required for picking up a full carrier including processed substrates from the load port and putting a carrier onto the load port to provide new substrates to be processed, does not substantially depend on the lot size, whereas the time window for the opportunity to perform an actual carrier exchange is highly dependent on the respective lot size, since a small currently processed lot provides only a reduced time interval for exchanging another carrier without producing an undesired idle time, also referred to as a window of opportunity for carrier exchange. Thus, the presence of a mixture of lot sizes, such as pilot lots, development lots and the like, or the presence of lots having a high priority, may negatively affect the overall performance of process tools.
Moreover, in view of cycle time enhancement for the individual products and to address flexibility in coping with customers' specific demands, the lot size may decrease in future process strategies. For example, currently 25 wafers per transport carrier may be a frequently used lot size, wherein, however, many lots may have to be handled with a lesser number of wafers due to the above requirements, thereby imposing a high burden on the process capabilities of the automatic transport system and the scheduling regime in the facility in order to maintain a high overall tool utilization. That is, the variability of the carrier exchange times for exchanging the carriers with respective load stations of the process tools may be high and thus a significant influence of the transport status in the manufacturing environment on the overall productivity may be observed. Thus, when designing or re-designing a manufacturing environment, for instance by installing new or additional equipment, the tool characteristics with respect to transport capabilities, such as the number of load ports for specific tools and the like, and the capabilities and operational behavior of the automatic material handling system (AMHS), may represent important factors for the performance of the manufacturing environment as a whole. The handling of small and different lot sizes within the manufacturing environment that is designed for a moderately large standard lot size may therefore require highly sophisticated scheduling regimes to compensate for the lack of sufficient carrier exchange capacity in the existing tools. However, the presence of small lot sizes may nevertheless result in a significant reduction of tool utilization, in particular for high throughput tools, as previously explained, due to the fact that a small or negative carrier exchange time associated with the processing of small lot sizes may not be compensated for, unless the number of load ports is significantly increased, as will be described with reference to FIGS. 1a(a)-1c(a) and 1a(b)-1c(b) in more detail.
FIG. 1a(a) schematically illustrates a top view of a portion of a manufacturing environment 150 and FIG. 1a(b) illustrates a corresponding timing diagram for operating the manufacturing environment 150. The manufacturing environment 150 may comprise a process tool 160, which is assumed to represent a process of high throughput with a rate of 500 substrates per hour. For instance, the process tool 160 may represent any appropriate cluster tool having a plurality of process chambers for performing a specified manufacturing process, such as an etch process, a deposition process and the like. The process tool 160 comprises a load port assembly 161 comprising a plurality of load ports LP1, LP2, LP3, LP4 which are configured to exchange substrate carriers 173 with an automated transport system 170. The transport system 170 is provided as an overhead transport system (OHT) comprising one or more rails 171, which are attached to the ceiling of a clean room (not shown) as is typically provided in the manufacturing environment 150 for fabricating semiconductor devices. Furthermore, appropriate transport vehicles 172 are provided which are configured to load and unload a substrate carrier, such as the carrier 173, at a respective one of the load ports LP1, LP2, LP3, LP4. It should be appreciated that the vehicles 172 typically comprise any appropriate mechanical components, such as grippers and the like, which enable the vehicles 172 to pick up the substrate carrier 173 from a load port by hoisting a carrier up to the vehicle 172 and also by hoisting down the carrier 173 at a destination load port. Moreover, typically, the transport system 170 is designed as a ring-like transport system, wherein a specified transport direction is determined for each process tool in the manufacturing environment 150. For instance, in the example of FIGS. 1a(a) and 1a(b), the transport direction may be from the right to the left.
During operation of the manufacturing environment 150, the high throughput of the process tool 160 causes a large flow of substrate carriers 173 to the load port assembly 161 to provide the required substrates to be processed in the tool 160, while the same flow of substrate carriers 173 also has to be maintained to remove the substrates that have already been processed in the tool 160. For example, if a substantially continuous operating mode is to be established with the maximum throughput of the tool 160, which is desirable in view of efficiency, about 42 carriers 173 per hour have to be picked up by the load port assembly 161. For a typical carrier exchange time, i.e., the time required for the transport system 170 to pick up a single transport carrier from a specific load port and to deliver a new transport carrier to the same load port, in the range of 6-10 minutes, one load port of the assembly 161 may support the delivery of 6-10 carriers 173 per hour and the pickup of 6-10 carriers per hour.
FIG. 1a(b) schematically illustrates a timing diagram corresponding to a process scenario as described above. That is, at each of the load ports LP1, LP2, LP3, LP4, a sequence of time intervals t1 and t2 would be required for a substantially continuous operational mode. In the example shown, the time interval t1 may correspond to a time interval during which substrates are loaded into a process module of the tool 160 and are subsequently being received by the substrate carrier, wherein respective small time intervals of approximately 0.1 minute may also be included in t1, which corresponds to carrier handling activities, such as a time interval td for docking the carrier 173 to the respective load port and opening a carrier door, if, for instance, it is provided in the form of a front opening unified pod (FOUP). Similarly, a time interval tu for closing the door and undocking the carrier may be included in the time interval t1. The time interval t2 may then represent the time available for the transport system 170 to pick up a respective carrier and position a new substrate carrier including substrates to be processed in the tool 160. The corresponding time interval t2 may also be referred to as carrier exchange time. As is evident from FIG. 1a(b), a slight overlap of the time intervals t1 at each of the respective load ports LP1, LP2, LP3, LP4 is required to ensure a continuous operation. Thus, prior to the expiration of the time interval t1 on load port LP4, a new carrier has to be supplied to the load port LP1 so that the corresponding carrier exchange time t2 is substantially determined by the overall process time for a respective carrier, which therefore depends on the number of substrates contained therein, for a given throughput of the process tool 160. In the process scenario described above, it may be assumed that a “small lot size” process mode has to be established in the manufacturing environment 150 by using, for instance, 12 substrates per carrier. In this case, the overall process time for one carrier may be 1.7 minutes, thereby resulting in a carrier exchange time of 3.6 minutes. Since a respectively short carrier exchange time may not be supported by the transport system 170, conventionally the number of load ports is increased.
FIG. 1b(a) schematically illustrates the manufacturing environment 150, wherein the process tool 160 may have the load port assembly 161 with 5 load ports LP1, LP2, LP3, LP4, LP5. Consequently, the width of the process tool 160, at least a front end portion thereof, may have to be increased to accommodate the increased number of load ports. Thus, an increased amount of clean room space may have to be provided.
FIG. 1b(b) schematically illustrates the situation during a continuous operation under the above-specified conditions. As is evident from the timing diagram of FIG. 1b(b), the time interval t2, i.e., the carrier exchange time, may increase to 5.0 minutes, which, however, may still not be within the capabilities of the transport system 170.
FIG. 1c(a) schematically illustrates a situation in which the process tool 160 comprises six load ports LP1, LP2, LP3, LP4, LP5, LP6, thereby even further increasing the width of the tool 160 and thus, the required floor space in the clean room. As shown in FIG. 1c(b), the carrier exchange time t2 may increase to 6.5 minutes, which may be within the capabilities of the transport system 170, thereby enabling a substantially continuous operation of the tool 160.
As a consequence, according to conventional strategies, support of processing of small lot sizes may require a significant increase of the number of load ports, which may, however, require substantial modifications of existing manufacturing environments or which may not even be compatible with existing layouts of clean rooms due to the non-available additional floor space. If an enhanced flexibility with respect to process regimes may be required in newly designed manufacturing environments, a sufficient high number of load ports may have to be taken into consideration when designing the manufacturing environment, which may add significant additional costs, since the clean room area has to initially be adapted to the desired degree of flexibility, i.e., to the minimum lot size, for which a substantially continuous operation of high throughput tools is desired.
The present disclosure is directed to various systems and techniques that may avoid, or at least reduce, the effects of one or more of the problems identified above.